The present invention relates to beam alignment systems, and more particularly to a beam alignment signal processing scheme that may be used to process the position signals associated with the beams being aligned by the beam alignment system where one of the beams remains on all the time.
In an optical disk storage system, data is stored by marking a rotating disk with a beam of radiant energy (typically a laser beam) that is modulated in some fashion by the data to be stored. To write or store data on the disk, the modulated beam is directed to and focused at a desired point (termed the "write point" for purposes of this application) on the surface of the disk. As the disk rotates under the write point, a "data track" is created by the marks made on the disk by the modulated beam. If the write point is held stationary, a circular data track is created centered about the axis of rotation of the rotating disk. Additional data tracks, each concentric with the others, can be created by blanking the write beam off, moving the write point radially with respect to the disk to a new location, holding the write point stationary at this new location, and turning the modulated write beam back on. Alternatively, if the write point is radially moved with respect to the disk as the modulated write beam makes marks thereon, a spiralling data track is created on the surface of the disk.
Whether the data tracks are concentric or spiralling, the available surface area on the disk is most efficiently used when the data tracks are spaced together as close as possible. The radial distance between adjacent data tracks is called the "track pitch". Accurately maintaining the track pitch at a desired value, especially where the track pitch must be kept small so as to efficiently make use of the storage space available on the disk, has presented a significant obstacle in the development of high storage capacity optical disk storage systems. Beam alignment systems are used to properly align a write beam with a read beam so that a desired track pitch can be maintained. Such systems align a write beam with a read beam that is following a previously written data track. Hence, as a new data track is written by the write beam, it will be aligned (have a desired track pitch) with the previously written track that the read beam is following.
A significant problem associated with such beam alignment systems is in the generation of a position signal or signals that indicate the relative alignment between the two beams. Such alignment is usually measured with a detector that is placed in the beam path, or that has split-off portions of the beams to be aligned directed thereto. However, a problem still exists in ascertaining the true position of a beam incident on the detector, regardless of the intensity of the incident beam or the number of beams that are present. A related problem exists where the detector has a plurality of beams incident thereto and one of the incident beams may not be turned off without turning off all of the beams.